Differentiate between upward and downward multiplexing?

The process of multiplexing is carried out at the transport layer. Several conversations are multiplexed in to one connection or physical links or virtual circuit. For example, suppose the host has only one network address available for use. Then it has to be used by all the transport connections originating at that host. For multiplexing the following two main strategies are followed:

Ø  Upward multiplexing and

Ø  Downward multiplexing

Upward Multiplexing 

- In upward multiplexing, the different transport connections are multiplexed in to one network connection. 

- These transport connections are grouped by the transport layer as per their destinations. 

- It then maps the groups with the minimum number of network connections possible.

- The upward multiplexing is quite useful where the network connections come very expensive.

Downward Multiplexing 

- It is only used when the connections with high bandwidth are required. 

- In case of the downward multiplexing, the multiple network connections are opened by the transport layer and the traffic is distributed among them. 

- But for using downward multiplexing, it is necessary that this capacity must be handled well by the subnet’s data links.

Another Technique 

- In either of the cases it is not guaranteed that the segments will be delivered in order. 

- Therefore, another technique is adopted. 

- The segments are numbered sequentially. 

- Each octet is numbered by the TCP sequentially. 

- Segments are then numbered based up on the number of the first octet present in that segment. 

- The segments might get damaged in the transition or some may even fail to arrive at the destination. 

- This failure is not acknowledged by the transmitter. 

- However, the successful receipt of the segment is does acknowledged by the receiver. 

- Sometimes, the cumulative acknowledgements might be used. 

- If the ACK triggers a time out interrupt, the re-transmission of the segment is done. 

- Also the re-transmission is done when an ACK is lost. 

- The receiver must have the ability to recognize the duplicate ACKs. 

- If such thing occurs, the receiver assumes by itself the ACK might have been lost.

- This happens when the ACK duplicate is received before the connection is closed. 

- If the duplicate is received after the closure of the connection, the situation is dealt differently. 

- In this case, the sender and receiver are allowed to know about each other’s existence. 

- They negotiate about the parameters and the transport entity resources are allocated based up on some mutual agreement. 

The connection release is of two types:

Ø Asymmetric release: 

This is the one used in the telephone systems. However it does not works well for the network that use packet switching.

Ø  Symmetric release: 

- This is certainly better than the previous one.

- Here, all the directions are released independently with respect to each other. 

- The host continues receiving data after the disconnection TPDU has been sent. 

- But the symmetric release has another problem which is related with indirection levels and fake messages. 

- There are no proper solutions for this problem in case of the unreliable communication media. 

- Note that this has nothing to do with the protocol. 

- Putting a reliable protocol over an unreliable medium can actually guarantee the delivery of the message. 

- Another thing to be noted is that it the time limit within which the message will be delivered cannot be guaranteed by any protocol. 

- Error conditions might prolong the delivery period. 

- Restarting the connections can lead to the loss of all the state info and the connection might remain as half-open. 

- Since no protocol has been designed to deal with this problem therefore one has to go forward with the risks associated with releasing the connections. 

What are the advantages and disadvantages of datagram approach?

- Today’s packet switching networks make use of a basic transfer unit commonly known as the datagram. 

- In such packet switched networks, the order of the data packets arrival, time of arrival and delivery comes with no guarantee. 

- The first packet switching network to use the datagrams was CYCLADES. 

- Datagrams are known by different names at different levels of the OSI model. 

- For example, at layer 1 we call it Chip, at layer 2 it is called Frame or cell, data packet at layer 3 and data segment at layer 4. 

- The major characteristic of a datagram is that it is independent i.e., it does not rely on any other thing for the information required for exchange.

- The duration of a connection between any two points is not fixed such as in telephone conversations. 

- Virtual circuits are just the opposite of the datagrams. 

- Thus, a datagram can be called as a self containing entity. 

- It consists of information sufficient for routing it from the source to the destination without depending up on the exchanges made earlier. 

- Often, a comparison is drawn between the mail delivery service and the datagram service. 

- The user’s work is to just provide the address of the destination. 

- But he/she is not guaranteed the delivery of the datagram and if the datagram is successfully delivered, no confirmation is sent to the user. 

- The data gram are routed to some destination without help of a predetermined path. 

- The order in which the data has to be sent or received is given no consideration. 

- It is because of this that the datagrams belonging to a single group might travel over different routes before they reach their common destination. 

Advantages of Datagram Approach

1. Datagrams can contain the full destination address rather than using some number.
2. There is no set up phase required for the datagram circuits. This means that no resources are consumed.
3. If it happens during a transmission that one router goes down, the datagrams that will suffer will include only those routers which would have been queued up in that specific router. The other datagrams will not suffer.
4. If any fault or loss occurs on a communication line, the datagrams circuits are capable of compensating for it.
5. Datagrams play an important role in the balancing of the traffic in the subnet. This is so because halfway the router can be changed.

Disadvantages of Datagram Approach

1. Since the datagrams consist of the full destination address, they generate more overhead and thus lead to wastage of the bandwidth. This in turn makes using datagram approach quite costly.
2. A complicated procedure has to be followed for datagram circuits for determining the destination of the packet.
3. In a subnet using the datagram approach, it is very difficult to keep congestion problems at bay.
4. The any-to-any communication is one of the key disadvantages of the datagram subnets. This means that if a system can communicate with any device, any of the devices can communicate with this system. This can lead to various security issues.
5. Datagram subnets are prone to losing or re - sequencing the data packets during the transition. This puts a great burden on the end systems for monitoring, recovering, and reordering the packets as they were originally.
6. Datagram subnets have less capability of dealing with congestion control as well as flow control. This happens because the direction of the incoming traffic is not specified. In the virtual circuit subnets, the flow of the packets is directed only along the virtual circuits thus making it comparatively easy for controlling it.
7. The unpredictable nature of the flow of the traffic makes it difficult to design the datagram networks. 

What is inter-network routing?

In this article we shall discuss about inter-network routing. Before moving to that there are certain terms with which you should be familiar:

Ø  End systems: The ISO (the international standards of organization) defines the end systems as the network elements that do not have the ability of forwarding the packets across the networks. Sometimes the term host is used to refer to the end systems.

Ø  Intermediate systems: These are the network elements that have the ability of forwarding the packets across the network. Most common examples are routers, switches, bridges and so on.

Ø  Network: It can be defined as a part of the inter-network infrastructure encompassing various elements including hubs, repeaters, bridges and so on. The networks are bounded by the intermediate systems.

Ø Router: This is one of the intermediate systems that is used for connecting various networks with each other. It might support one protocol (router) or many protocols (multi-protocol router). Its hardware part is optimized especially for performing routing. The software part is responsible for carrying out the routing and takes care of the routing tables.

Apart from these devices, there are 3 types of addresses involved in inter-network routing:

Ø  The inter-network address: The host address and the network address are combined together to form this address. This is used for unique identification of a host over the inter-network.

Ø The host address or host ID: This ID might be assigned by the administrator or might be simply the physical address of the host. It is used for the unique identification of the host on its network.

Ø  The network address or network ID: This is address of a network for identifying it in an inter-network.

All the data packets consist of a network layer header. This network layer header consists of the following when the packet is transmitted from one host to another:

ØThe address of the source inter-network: This address combines the address of the source host and the source network.

ØThe address of the destination inter-network: This address combines the address of the destination host and the destination network.

ØThe hop count: This usually begins at zero and is numerically incremented when the packet crosses a router. Or in the opposite case it might be assigned some maximum value which might be decremented on reaching a router. The purpose of using the hop count is to make sure that the packet does not keeps on circulating endlessly in the network.

- For inter-network routing, two things have to be known.

- Firstly, how do you reach other routers which lie in the same network and secondly, how do you reach other routers which lie in other networks? 

- The answer to the first question is easy as it is the common routing problem among two hosts residing over the same network. 

- This routing is handled by the interior gateway protocol and it is different for different networks since only local routing info is required. 

- In this case, the commonly used protocol is the open shortest path first or OSPF protocol. 

- The routing between two different networks is performed using the exterior gateway protocol. 

- This is actually the problem of inter-network routing. 

- Here, the commonly used protocol is the BGP or the border gateway protocol. 

- The graph for inter-network routing is quite different from the one that is used in the network routing. 

- This is so because the routers which lie in the same network can be thought of as being directly connected to one another for routing across inter-network. - All the networks in an inter-network function as though they are one large unit. 

What are the differences between bridges and repeaters?

Bridges and repeaters are both important devices in the field of telecommunications and computer networking. In this article we discuss about these two and differences between them. 

- The repeaters are deployed at the physical layer whereas one can find bridges at the MAC layer. 

- Thus, we called repeaters as the physical layer device. 

- Similarly, bridge is known as the MAC layer device. 

- Bridge is responsible for storing as well forwarding the data packets in an Ethernet.

- Firstly, it examines the header of the data frame, selects few of them and then forwards them to the destination address mentioned in the frame. 

- Bridge uses the CSMA/CD for accessing a segment whenever the data frame has to be forwarded to it.

- Another characteristic of a bridge is that its operation is transparent. 

- This means that the hosts in the network do not know that the bridge is also present in the network. 

- Bridges learn themselves; they do not have to be configured again and again. 

- They can be simply plugged in to the network. 

- Installing a bridge causes formation of LAN segments by breaking a LAN. 

- Packets are filtered with the help of bridges. 

- The frames that belong to one LAN segment are not sent to the other segments. 

- This implies separate collision domains are formed. 

The bridge maintains a bridge table consisting of the following entries:

1. LAN address of the node
2. Bridge interface
3. Time stamp
4. Stale table entries

- Bridges themselves learn that which interface can be used for reaching which host. 

- After receiving a frame, it looks for the location of the sending node and records it.

- It keeps the collision domains isolated from one another thus, giving the maximum throughput. 

- It is capable of connecting a number of nodes and offer limitless geographical coverage. 

- Even different types of Ethernet can be connected through it. 

- Even the repeaters are plug and play devices but they do not provide any traffic isolation. 

- Repeaters are used for the purpose of regenerating the incoming signals as they get attenuated with time and distance. 

- If physical media such as the wifi, Ethernet etc. is being used, the signals can travel only for a limited distance and after that their quality starts degrading. 

- The work of the repeaters is to increase the extent of the distance over which the signals can travel till they reach their destination. 

- Repeaters also provide strength to the signals so that their integrity can be maintained. 

- Active hubs are an example of the repeaters and they are often known as the multi-port repeaters. 

- Passive hubs do not serve as repeaters. 

- Another example of the repeaters are the access points in a wifi network. 

- But it is only in repeater mode that they function as repeaters. 

- Regenerating signals using repeaters is a way of overcoming the attenuation which occurs because of the cable loss or the electromagnetic field divergence. 

- For long distances, a series of repeaters is often used. 

- Also, the unwanted noise that gets added up with the signal is removed by the repeaters. 

- The repeaters can only perceive and restore the digital signals.

- This is not possible with the analog signals. 

- Signal can be amplified with the help of amplifiers but they have a disadvantage which is that on using the amplifiers, the noise is amplified as well. 

- Digital signals are more prone to dissipation when compared to analog signals since they are completely dependent up on the presence of the voltages. 

- This is why they have to be repeated again and again using repeaters. 

Explain the technique of admission control to control congestion in virtual circuit subnets?

- Virtual circuits can be thought of as a virtual channel in the telecommunication networks as well as computer networks. 

- Virtual circuit sub-nets represent a communication service that is connection oriented. 

- This service is used through the packet mode communication.

- A stream of data bytes can be exchanged between the two nodes only if a virtual circuit has been established between them. 

- Without the presence of the higher level protocols, data division has to be dealt with unnecessarily. 

- Therefore, the virtual circuits always allow the high level protocols. 

- There is a resemblance between the circuit switching mode and the virtual circuits because of the fact that both of them are based up on connection. 

- The packets that are transmitted through a virtual circuit sub-net consist of a circuit number and not the destination address. 

- This is why the memory requirement of the packets is less when using virtual circuits and more in the others. 

- This also makes the virtual circuit sub nets less expensive when compared to other sub nets. 

In this article we discuss about the technique for congestion control in virtual circuit sub nets. 

- One most popular technique is of the admission control. 

- Most of the congestion control methods are based up on an open loop i.e., the congestion is prevented rather than managing it after it has occurred. 

- Admission control is a dynamic method for controlling the congestion problems in the virtual circuit sub nets. 

- Admission control technique has been widely accepted for preventing the congestion problem from getting worse over the time. 

- The technique is based up on a very simple idea which is that no virtual circuit is set up until and unless the congestion problem that was detected has been resolved. 

- Therefore, any attempt that is made for establishing a new virtual connection with the transport layer is failed. 

- The things get even worse if the access is granted to more and more people. 

- Simplicity of the technique is one of its characteristic that makes it easy to be implemented. 

- The technique can be implemented in a straightforward manner. 

- The admission control technique is also used by the telephone systems for combating with the congestion problems. 

- The admission control technique is implemented whenever a switch in the network gets overloaded. 

- At this time no dial tone is heard. 

- Establishing new virtual connections represents another way of coping with this problem. 

- Here, the new connections have to be routed carefully such that there are no problems. 

Another method for eliminating the problem of congestion is to strike an agreement between the virtual circuit subnet and the host. 

- By this we mean establishing a new virtual circuit. 

- But this arrangement requires specifying how the traffic has to be shaped and what would be its volume, QoS (quality of service), other parameters and so on. 

- The virtual circuit sub net has to reserve the resources on its part of the agreement established. 

- These resources lie on the route where the virtual circuit has been established. 

- The resources might include the following:

Ø  Space in the router’s buffer.

Ø  Tables

Ø  Bandwidth of the lines and so on.

- The newly virtual circuits are less likely to experience the congestion problems. 
- This is because to them the availability of the resources has been guaranteed.
- Resources can be reserved in this way only if the existing sub nets are experiencing congestion problem or when the standard operating procedure is being followed. 
- One disadvantage of the admission control technique is that it leads to the wastage of the resources. 
- Also, sometime the bandwidth is left unused. 

What is a choke packet?

- The networks often experience problems with congestion and flow of the traffic. 

- While implementing flow control a special type of packet is used throughout the network. 

- This packet is known as the choke packet. 

- The congestion in the network is detected by the router when it measures the percentage of the buffers that are actually being used. 

- It also measures the utilization of the lines and average length of the queues. 

- When the congestion is detected, the router transmits choke packets throughout the network. 

- These choke packets are meant for the data sources that are spread across the network and which have an association with the problem of congestion. 

- These data sources in turn respond by cutting down on the amount of the data that they are transmitting. 

- A choke packet has been found to be very useful in the maintenance tasks of the network. 

- It also helps in maintaining the quality to some extent. 

- In both of these tasks, it is used for informing the specific transmitters or the nodes that the traffic they are sending is resulting in congestion in the network. 

- Thus, the transmitters or the nodes are forced to decrease the rate at which they are generating traffic. 

- The main purpose of the choke packets is controlling the congestion and maintaining flow control throughout the network. 

- The router directly addresses the source node, thus causing it to cut down its data transmission rate. 

- This is acknowledged by the source node by making reductions by some percentage in the transmission rates. 

- An example of the choke packet commonly used by the most of the routers is the source quench packet by ICMP (internet control message protocol).  

- The technique of using the choke packets for congestion control and recovery of the network involves the use of the routers. 

- The whole network is continuously monitored over by the routers for any abnormal activity.

- Factors such as the space in the buffers, queue lengths and the line utilization are checked by the routers. 

- In case the congestion occurs in the network, the choke packets are sent by the routers to the corresponding parts of the network instructing them to reduce the throughput. 

- The node that is the source of the congestion has to reduce its throughput rate by a certain percentage that depends on the size of the buffer, bandwidth that is available and the extent of the congestion. 

- Sending the choke packets is the way of routers telling the nodes to slow down so that the traffic can be fairly distributed over the nodes. 

- The advantage of using this technique is that it is dynamic in nature. 

- The source node might send as much data as required while the network might inform that it is sending large amounts of traffic.

- The disadvantage is that it is difficult to know by what factor the node should reduce its throughput.

- The amount of the congestion being caused by this node and the capacity of the region in which congestion has occurred is responsible for deciding this. 

- In practical, this information is not instantly available. 

- Another disadvantage is that after the node has received the choke packet, it should be capable of rejecting the other choke packets for some time. 

- This is so because many additional choke packets might be generated during the transmission of the other packets. 

The question is for how long the node is supposed to ignore these packets? 

- This depends up on some dynamic factors such as the delay time. 

- Not all congestion problems are same, they vary over the network depending up on its topology and number of nodes it has. 

What is meant by flow specification?

- There are many problems concerning the flow specification. 

- There are limited options for the provider for mitigation of the DDoS attacks that take place internally. 

- These can be categorized in to three different categories:

Ø  BGP (border gateway protocol) destination black holes

Ø  BGP src/ uRP

Ø  ACLS

- The basic idea is to make use of the BGP for the distribution of the flow specification filters. 

- This helps in dynamic filtering in the routers. 

- The flow specification rules are encoded according to the BGP NLRI address family. 

- The flow spec NLRI is used by the BGP as its opaque key is used as an entry key for its database. 

- The extended communities are used for specifying the actions such as accepting, discarding it, rate limiting, sampling, redirecting and so on. 

- The source/destination prefix and the source/destination port are matched in combinations according to the packet size, ICMP type/co9de, fragment encoding, DSCP, TCP flag and so on. 

- For example, the TCP ports 80…90 are matched with 192.168.0/24. 

- The flow specification trust model uni casts the routing advertisements for controlling the traffic. 

- Filter is considered as a hole for the traffic that is being transmitted to some destination. 

- Filter is accepted when it is advertised for the destination by the next hop. 

- Filters with various flow specifications are available today.

- The major benefit of the flow specifications is the filters with the fine grain specification which make it easy for deploying and managing the BGP. 

- The trust and the distribution problems are solved by the BGP. 

- ASIC filtering in routers is leveraged. 

- This is another major benefit of flow specifications. 

Apart from the benefits, there are various limitations of the flow specifications as mentioned below:

Ø  There is no update level security in the BGP.

Ø The statistics and the application level acknowledgement are not well defined.

Ø  The flow specifications work only for those nodes for which the BGP has been enabled.

Ø  Beyond routing the BGP payload has to be overloaded.

Ø  There are various operational issues between the security operations and the network operations.

Ø  The threat information cannot be gathered in one place.

- The integration of the flow specifications was announced by various security vendors. 

- The DDoS attacks are experienced by a large number of customers. 

- The DDoS attacks are now massive and have put the network infrastructure at risk apart from the end customer. 

- Congestion problems occur at both the exchange and the backbone. 

- The attacks of long durations add to the cost of bursting and circuit congestion problems. 

- Depending up on the size of the attack the POP has to be isolated.

- VoIP is also affected. 

- These attacks have negative economic effects as the cost of the operations has been increased. 

- This has led to a degradation of the business. 

- Measures such as firewall filtering and destination BGP black-holing have proved to be insufficient in preventing the attacks. 

- These methods are slow since it is required to log-in and configuring the devices. 

- The configuration has to be constantly. 

- The traffic is terminated to some destination. 

- This affects the availability. 

- The black hole routes are removed by constantly changing the configurations. - Earlier version of the flow specifications had many bugs. 

- There were some limitations on the performance. 

- However, it provided arbor support for the actions of the flow specifications. 

- It does not provide multi–vendor support. 

- To some extent it provides the mitigation facility for the attack that occurred at the source. 

- The collateral damage is eliminated for both the carriers and supports the change in the matching criteria. 

Explain multicast routing?

- Multicast routing is also known as the IP multicast. 

- For sending the IP (internet protocol) data-grams to a group of receivers who are interested in receiving the data-grams, multicast routing is used.

- The data-grams are sent to all the receivers in just one transmission. 

- Multicast routing has got a special use in the applications that require media streaming on private networks as well as internet. 

- Multicast routing is IP specific version. 

- A more general version is the multicast networking.

- Here, the multicast address blocks are especially reserved in IPv6 and IPv4. 

- Broadcast addressing has been replaced by multicast addressing in IPv6. 

- Broadcast addressing was used in IPv4. 

- RFC 1112 describes the multicast routing and in 1986 it was standardized. 

This technique is used for the following types of real – time communication over the IP infrastructure of the network:

Ø  Many – to – many

Ø  One – to – many

- It scales up to receiving population that is large enough and it does not require either knowledge regarding the receivers and the identity of the receivers. 

- Network infrastructure is used efficiently by the multicast efficiently and requires source sending packet to a large number of receivers only once. 

- The responsibility of the replication of the packet is of the nodes which are nothing but the routers and the network switches.

- The packet has to be replicated till it reaches the multiple receivers. 

- Also, it is important that the message is sent only once over the link.   

- UDP or the user data gram protocol is the mostly used protocol of low level. 

- Even though if this protocol does not guarantees reliability i.e., the packets might get delivered or get lost. 

- There are other multicast protocols available that are reliable such as the PGM or the pragmatic general multicast. 

It has been developed for adding the following two things a top the IP multicast:

Ø  Retransmission and

Ø  Loss detection

The following 3 things are key elements of an IP multicast:

1. Receiver driven tree creation
2. Multicast distribution tree
3. IP multicast group address

- The receivers and the sources use the last for sending as well as receiving the multicast messages. 

- The group address serves as the destination address of the data packets for the sources whereas it is used for informing the network whether or not the receivers want those packets.

- Receivers need a protocol for joining a group. 

- One most commonly used protocol for this purpose is the IGMP i.e., the internet group management protocol. 

- The multicast distribution trees are set up using this protocol. 

- Once a group has been joined by the receiver, the PIM (protocol independent multicast) protocol is used for constructing a multicast distribution tree for this group. 

- The multicast distribution trees set up with the help of this protocol are used for sending the multicast packets to the members of the multicast group. 

PIM can be implemented in any of the following variations:

1. SM or sparse mode
2. DM or dense mode
3. SSM or source specified mode
4. SDM or sparse – dense mode or bidirectional mode (bidir)

- Since 2006, the sparse mode is the most commonly used mode. 

- The last two variations are more scalable and simpler variations of PIM and are also popular. 

- An active source is not required for carrying out an IP multicast operation and knowing about the group’s receivers. 

- The receiver drives the construction of the IP multicast tree. 

- The network nodes which lie closer to receiver are responsible for initiating this construction.

- This multicast then scales to a receiver population that is large enough. 

- It is important for a multicast router to know which all multicast trees can be reached in the network. 

- Rather, it only requires knowledge of its downstream receivers. 

- This is how the multicast – addressed services can be scaled up.